1. Field of the Invention
The present invention relates to a pyrazine derivative. The present invention also relates to a light emitting element containing the pyrazine derivative and a display device that includes a light emitting element containing the pyrazine derivative.
2. Description of the Related Art
In recent years, a light emitting element using a light emitting compound has been attracted attention as a display (a display device) of the next generation because it has a feature of low power consumption and a lightweight and thin type. In addition, the light emitting element using a light emitting compound is a self-luminous type; accordingly, it is considered that the light emitting element using a light emitting compound has superiority in visibility without problems such as a viewing angle as compared with a liquid crystal display (an LCD).
A basic structure of a light emitting element is a structure that has a light emitting layer containing a light emitting compound interposed between a pair of electrodes. It is said that, in such a light emitting element, by applying a voltage, holes injected from an anode and electrons injected from a cathode are recombined in an emission center of a light emitting layer to excite a molecule, and the excited molecule discharge energy in returning to a ground state; accordingly light is emitted. It is to be noted that an excited state that is generated by recombination has a singlet excited state and a triplet excited state. Light emission is considered to be possible through a singlet excited state and a triplet excited state. In particular, light emission in a case of returning from the singlet excited state to the ground state directly is defined as fluorescence, and light emission in a case of returning from the triplet excited state to the ground state is defined as phosphorescence.
It is considered that the singlet excited state and the triplet excited state, which are an excited state, are generated at a ratio of 1:3 statistically. Accordingly, when phosphorescence that is light emission in a case of returning from the triplet excited state to the ground state is used, it is theoretically considered that a light emitting element having internal quantum efficiency (a ratio of photon that is generated with respect to injected carriers) of 75 to 100% can be obtained. That is to say, if phosphorescence can be utilized, light emitting efficiency can be remarkably improved as compared with utilizing fluorescence.
However, phosphorescence can not be observed at a room temperature in a general organic compound. This is because that the ground state of an organic compound is ordinarily in the singlet ground state, and transition from the triplet excited state to the singlet ground state becomes forbidden transition. On the other hand, transition from the singlet excited state to the singlet ground state becomes allowed transition, and therefore, fluorescence can be observed. However, in recent years, a compound capable of emitting phosphorescence, in other words, a compound capable of converting light in returning from the triplet excited state to the ground state into light emission (hereinafter, referred to as a phosphorescent compound) is discovered as shown in Patent Document 1, and it has been actively researched (for example, see Patent Document 1: Japanese Published Patent Application No. 2005-170851).
When a light emitting element is manufactured using a phosphorescent compound, the phosphorescent compound is used in a state where the phosphorescent compound is dispersed in a host material in order to prevent decrease of light emitting efficiency due to concentration quenching. Therefore, in order to efficiently obtain light emission from the phosphorescent compound, selection of the host material becomes important.
In order to efficiently obtain light emission from the phosphorescent compound, it is found that a host material having a bipolar property is suitable. However, many of organic compounds are a material having a monopolar property, which has either a hole transporting property or an electron transporting property. Therefore, a material having a bipolar property, which has both the hole transporting property and the electron transporting property, is required to be developed.